This invention relates to optical filters.
Optical band pass filters have generally been of the interference type in which a succession of layers of materials are deposited on a substrate. Such filters function as band pass filters because constructive interference occurs between reflections for a certain narrow range of wavelengths permitting transmission of these wavelengths, while for other wavelengths interference is not constructive and little or no light is transmitted. Interference filters can for all practical purposes be made to transmit any desired wavelength with as narrow a pass band as is desired by properly selecting the thickness and the number of layers deposited. They are effective, however, only when used to filter well columnated light since the favorable interference condition is influenced by the angle of incidence. A further drawback is that the band pass is subject to considerable variability due to thermal expansion.
Absorption type filters which function by atomic or molecular absorption throughout the bulk of the filter material are not subject to much dependence on the angle of incidence, but they cannot be arbitrarily designed to operate at any wavelength and they have not generally provided narrow pass bands.